For some time now, some of my on-line friends have advised me to provide a version of CassidySlangScam without the invective aimed at Cassidy and his supporters. In response to that advice, I am working on providing a glossary of the terms in Cassidy’s ludicrous book How The Irish Invented Slang with a short, simple and business-like explanation of why Cassidy’s version is wrong.
Cassidy, in his work of etymological fiction, How The Irish Invented slang, claims that this word comes from some Irish root related to the word meall meaning to entice or deceive. Cassidy points to the words meallacach and meallaireacht.
Interestingly, this website here (http://www.oldknot.com/2012/10/the-malarkey-about-malarkey/) tells us that the link with meallaireacht was first proposed by a certain William Sayers in 2002, when Cassidy was hitting the internet searching for potential Irish-origin words. In other words, this seems to be another claim plagiarised by Cassidy from an internet source without acknowledgement.
There is no certainty about where malarkey or mullarkey comes from. There is certainly an Irish surname Mullarkey (Ó Maoilearca) and Joe Biden famously claimed the word as part of his Irish heritage a few years ago. Some linguists think that it is linked to a northern English dialect term, malark, meaning a prank. If it were really of Irish origin, you would expect it to be found in Irish English a long time before it occurs in America, and this does not seem to be the case.